The present invention relates to optical display systems. More specifically, the present invention relates to an optical display system including a ferroelectric light valve.
A typical liquid crystal display ("LCD") projection system includes a source of white light, a means for separating the white light into red, green and blue spectral components, and a light valve for spatially modulating each of the spectral components. Instead of using a single white light source, separate light sources may be used for each primary color (e.g., R,G, B or Y,M,C). Such sources could be, for example, LEDs or laser diodes. The light valve performs the spatial modulation in response to a video signal generated by a computer. Resulting are images in red, green and blue color planes. The color planes may be recombined and then projected onto a screen, or they may be flashed in sequence onto the screen at such a rate that a viewer perceives only a single image.
LCD projection technology is now being adapted for use in computer monitors, television sets and other devices that use cathode ray tubes. It is anticipated that display systems including light valves will replace systems including cathode ray tubes. If this happens, computer monitors, television sets and other devices will become more compact, lighter in weight, and less expensive.
A variety of different light valves could be used. Among the various light valves are transmissive light valves, reflective light valves, ferroelectric light valves, nematic twisted light valves, digital micro-machined mirror light valves, and diffraction grating-type light valves.
The reflective ferroelectric light valve spatially modulates the spectral components by polarizing the components in different directions. Light that is reflected by the ferroelectric light valve might be polarized vertically when the light valve is "on" and horizontally when the light valve is "off." The polarized light is reflected towards a polarizing beam splitter. Reflected light that is polarized vertically might be reflected by the polarizing beam splitter towards a viewing plane, while reflected light that has been polarized horizontally might be directed elsewhere. Thus, if all three color components of a given pixel are polarized vertically, that pixel might appear gray or white on the viewing plane. If all of the color components of a given pixel are horizontally polarized, that pixel will appear black on the viewing plane.
It is often desirable to have a high contrast in the image displayed on the viewing plane. That is, it is often desirable for the black colors to be as black as possible and the white colors to be as white as possible. A contrast ratio of 300:1 is considered very good; a contrast ratio of 20:1 is not considered good.
To make the black colors as black as possible, the beam splitter should reflect little or no light towards the viewing plane. Ideally, each spectral component would be fully polarized so as not to be reflected by the beam splitter towards the viewing plane.
In practice, however, it is difficult to polarize each component so that no light is reflected by the beam splitter towards the viewing plane. Some light is usually reflected by the beam splitter, even when it should not be. Consequently, the contrast of the image is reduced, and the quality of the viewable image is degraded.
Poor contrast has been a problem in optical display systems including reflective ferroelectric light valves. Consequently, image quality of optical display systems based on reflective ferroelectric light valves has suffered.
There is a need to improve the contrast ratio in optical display systems including reflective ferroelectric light valves.